The present invention relates to artificial heart valves being comprised of a valve case (stint) and one or several vanes or leaves arranged in the interior thereof.
Artificial heart valves have been made in the past with a variety of methods. In accordance with one approach, the valve is made through die casting or injection molding whereby the case and stint of the valve as well as the vane arranged in the interior of the case are made in one operating step. However, in accordance with the requirements for casting, the walls of any parts involved must have a certain minimum thickness. This, then, means that, on one hand, the valve case (stint) can be appropriately selected in accordance with its requirement as these requirements are such that the case wall will be thicker than the minimum thickness of the device. On the other hand, the vanes for the flaps are too thick when they have to meet the minimum thickness requirements. Accordingly, these vane and flap elements are in fact inordinately thick for satisfactory operation.
In accordance with a different method, the case for the valve is made separately from the vane and flap whereby the case, for example, is made as an annulus and the flap and vane is made separately to be connected to the case subsequently, for example, by means of bonding. This bonding by means of an adhesive is carried out manually. Moreover, it is almost inevitable that certain unevennesses appear in the transitions between vane and case. Moreover, certain adhesive residue may accumulate more or less at random. All these factors result in unevennesses of the surface of the several parts. The very fact that the material involved is inhomogenic and that the surfaces are not even, makes it inevitable that within a very short time certain deposits appear at these unevenesses which deposits are of a cellular nature and involve interalia components of the blood. As a consequence, calcification occurs subsequently and impedes the blood transmission up to a point of complete blockage.
It is an object of the present invention to provide a new and improved method for making heart valves which are homogenic from the point of view of the utilized material as well as from the point of view of surface contours whereby in particular the transition zone between the flap vanes and the case and stint does not exhibit any unevennesses that may lead to the deposit of blood residue.
In accordance with the preferred embodiment of the present invention, the following method is suggested. A core is provided as a die element having surfaces which correspond to the shape of the vanes for heart valve flaps, this core is dipped at a low speed of descent into a polymer solution having a high viscosity; after a film or coating has formed on these surfaces, the core is slowly withdrawn from the solution whereupon the core with polymer coating is dried; the foregoing i.e. the dipping and retracting is repeated several times. An independently made valve case is dipped into a polymer solution of low viscosity and held therein, the case having an interior diameter which is slightly larger than the diameter of the aforementioned core, following which the core with polymer layers is slowly inserted into the valve case and into the polymer solution contained therein, whereby upon inserting the core into the valve case, the displaced solution can continuously flow evenly from the interior of the case; subsequently the core as well as the valve case are slowly withdrawn from the solution after a period of time sufficient for the formation of a polymer coating upon the combined core and valve case configuration whereupon the core proper, following drying of the coating, is separated from the valve case and from the previously made coating, the latter having combined with the valve case coating. The viscosity of the first mentioned, i.e. high viscosity polymer solution, should be in the range from 24 to 192,000 cp and the viscosity of the low viscosity solution should be within the range from 1500 to 2000 cp. The core should have been highly polished prior to use or be provided with a coating that does not react with the polymer solution but makes sure that the surface of the thus coated core is very smooth. The polymer solution and agent is preferably a polyurethane solution.
The inventive method offers the advantage that the valve case (at least on the outside) and the flap vane are made from the same material. Moreover, the combined coating of core and valve case results in a coating that physically combines the coating for both of them so that inhomogenities and step transitions between them are avoided. In particular then there are no unevennesses formed in the surface which subsequently could establish loci for the depositing of calcium and phosphate. Moreover, the dipping method as proposed here permits the formation of very thin, film-like coatings which can be increased in thickness and width through multiple dippings. On the other hand, the valve vanes can be made of a very small wall thickness because the dipping method leads to material thickness well below the minimum thickness values that have to be observed during injection molding.
The preferred method as described above suggests the utilization of the same material for the flap vanes and the valve casing but as far as practising the method is concerned, this is not essential in principle. However, the final coating is carried out in the low polymer solution and involves the flap vanes as well as the casing in unison so that this final coating is homogenic in contour and material and it is also smooth and transitionless.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention, and further object features and advantages thereof will better understood from the following description taken in connection the accompanying drawings in which: